The present invention relates to a process, preferably a continuous process, of coating a substrate, which allows water vapour and preferably air permeation, with a hydrophobic coating. The process preferably uses a thin film vacuum condensation step to create a monomer coating which is cured in situ after the coating. The process has the benefit of allowing continuous operation and providing a alternative to existing processes for hydrophobic coating. The substrates are coated such that the water vapour permeation sites are not blocked by the coating to maintain the desired breathability. The static water contact angle on the surface of such substrates is more than 95xc2x0. Such substrates are preferably employed in absorbent articles such as diapers, incontinence products, underarm sweat pads, sanitary napkins, catamenials, pantiliners, breast pads, shoe inserts or bandages or alternatively in protective garments such as gloves or rain coats for which water vapour transmission in particular from the inside to the outside is highly desirable.
For instance, U.S. Pat. No. 3,498,527 teaches that paper board containers for liquids can be waterproofed by application of a waterproofing coating such as wax or polyethylene, and a similar method is shown in U.S. Pat. No. 2,708,645 for waterproofing paper drinking cups and in U.S. Pat. No. 3,212,697 for paper grocery sacks. In U.S. Pat. No. 3,597,313, temporary wet strength is imparted to paper by coating it with a polymeric alcohol-polymeric aldehyde reaction product. Coating processes, by themselves, have been used to produce disposable articles of sanitary clothing. In U.S. Pat. No. 3,078,849, a disposable sanitary napkin is disclosed which consists of an adsorbent layer having a liquid-repellent backing of polyvinyl alcohol or similar material capable of initially repelling water but eventually solubilizing. The degree of water-repellency, therefore the lifetime of the napkin, is controlled by varying the thickness of the backing. Because the necessary life of the napkin cannot be predicted by manufacturer or user, the backing must be sufficiently thick to take account of all normal contingencies. U.S. Pat. No. 3,542,028 is directed to a flushable sanitary napkin consisting of a cellulosic sheet treated with a fluoropolymer coating. U.S. Pat. No. 3,559,650 teaches the preparation of a sanitary napkin having two flush-disposable sides separated by a waterproof film too thin to support itself once both faces of the napkin have disintegrated upon disposal.
Analogous to the process of coating a surface with a waterproofing substance is the concept of reacting a surface with another material so as to form a reaction product on the surface which has water-repellent properties. For example, U.S. Pat. Nos. 2,130,212 and 3,137,540 teach that materials such as polymeric alcohols may be reacted with other materials to increase their water-repellent properties. The latter patent teaches treating polyvinyl alcohol articles with an aqueous emulsion of an aldehyde to impart water-repellency thereto. U.S. Pat. No. 3,626,943 teaches that disposable diapers can be made from polyvinyl alcohol and waterproofed on one side by reaction with formaldehyde. These reaction-type coating processes suffer from drawbacks. They are carried out in the aqueous phase which is complicated and requires relatively large quantities of reagents. Most of the processes which employ some form of in situ chemical reaction to produce a water-repellent surface are carried out in the liquid phase, some vapor phase treatments are taught by U.S. Pat. Nos. 2,306,222; 2,961,388; and 3,017,290. A known method of water and oil repellent finishing of textiles, described in U.S. Pat. No. 1,158,634, includes plasma treatment in a glow discharge in an atmosphere of inorganic gases, followed by treatment with a fluorine containing acrylic monomer in gas phase. Another prior method of achieving film plasma polymerization, described in U.S. Pat. No. 4,188,426, includes treatment in a glow discharge of per-fluoro-cyclo-butane or hexafluoroethane to reduce the friction coefficient and to improve the surface hydrophobia of organic and inorganic substrates (e.g. polyethylene films, metals). However these disclosures do not achieve a level of water repellency as the present invention when employing the coating process disclosed herein.
Plasma coating processes of metals, polymers, and other substrates, with fluorocarbon films are also known in the art. As an example, it is known from U.S. Pat. No. 4,869,922, that deposition from continuous (i.e. non modulated) radiofrequency (RF) glow discharges fed with fluorocarbons provides films, layers, tapes, plates, and differently shaped articles made of plastics, metals or other materials, with a thin fluorocarbon coating, with no other material interposed between the coating itself and the substrate. Such coatings are claimed to have very good adherence to the items processed, to be void-free, to be uniform, continuous and to show controlled wettability characteristics, which depend on their surface chemical composition. The non modulated, continuous plasma process of the above mentioned patent leads to coatings characterized by static water contact angle (WCA) values lower than 120xc2x0.
U.S. Pat No 5,328,576 discloses a method for imparting water and oil repellent surface properties to fabrics or paper that includes pretreatment in a low pressure oxygen plasma in the presence of water vapor followed by plasma polymerization of methane in a high frequency glow discharge carried out in the same treatment chamber. This method doesn""t deliver durable, permanent coatings with a WCA higher than about 120xc2x0.
U.S. Pat. No. 5,262,208 discloses an gas plasma treatment for archival preservation of paper manuscripts by a thin film protective polymer film. The treatment time is ranging from 30-3600 seconds. Other methods have been used to obtain thin coatings on the web materials with short treatment periods. Providing surface treatment is disclosed in U.S. Pat. Nos. 4,842,893 and 4,954,371 which describe a process for high speed coating of substrates with a complete and uniformly adhering layer and using electron beam radiation curing of the vapor deposited monomers for multilayer capacitators. U.S. Pat. No. 4,842,893 discloses high speed coating process including flash vaporization system and electron beam curing. Both of these electron beam disclosures are incorporated herein by reference. Other uses of electron beam coatings in the electronic industry field have been reported by Westinghouse science and technology center USA (Adv. Mat. Newsletter Volume 13, No 9, 1991 page 4).
While different water repellent treatments have been used in the past, such as monomer solution coating and curing, coating during laundering, plasma coating, there remains the need for having an other method that results in thin, preferably durable repellent coatings that can be obtained at high production speeds and with minimal changes in the substrate properties, such as flexibility, texture, comfort, and breathability. The present invention combines the advantages of the above mentioned high speed electron beam process for coating of a substrate with the benefit of producing an article with durable and water-repellent coating.
Thus although the materials employed in the practice of this invention are known in the art and are known in the context of water-repellent coatings, the process utilizing electron beam treatment for hydrophobic coatings provides an attractive alternative to the methods of the prior art.
In one aspect of the present invention, the method of using a high speed vacuum coating process for producing durable and thin water-repellent coatings on a substrate uses a movable support such as rotating drum in a vacuum chamber. The surface of the support is maintained at a temperature sufficient to permit condensation of a vaporized material deposited in the chamber. The material is a curable monomer with a relatively low molecular weight. The monomer vapor is created using a flash vaporizer. The desired amount of curable monomer is metered to a heated flash vaporizer system where the material is vaporized. It is then transported e.g. by it""s inherent pressure, to the substrate resting on the rotating drum and condensed on the surface of the substrate. According to the method the substrate is then transported to a curing means such as an energy source which emits an electron beam, UV-light radiation or exposure to an electro magnetic field. Alternative the curable monomer can also be transferred into radicals by passing through a plasma zone (zone of high voltage discharge). The curing of the monomer by the curing means then provides a coating on the substrate surface which has a static water contact angle of more than 95xc2x0.
The method for delivering the curable monomer to the substrate for minimizing the amount of monomers can use an ultrasonic atomizer producing micro droplets of curable monomer. They are released into a vaporization tube heated by band heaters. The atomized droplets impinge on the inner wall of the vaporization tube and are instantaneously vaporized, i.e., flash vaporized. This reduces the opportunity for polymerization prior to being deposited on the substrate.
In one aspect of the present invention, the substrate can be one side water-repellent and capable of absorbing and storing fluids from the other side, or alternatively be repellent on both sides.